The present invention is directed to the control of distortion during high temperature processing, and, more particularly, in the hot pressing of metallized multilayer ceramic (MLC) substrates.
In the manufacture of MLC substrates, ceramic greensheets are formed from a casting slurry. The individual ceramic greensheets are personalized with via holes and conductive metal. The ceramic greensheets are then stacked together in a predetermined design sequence to form a green ceramic laminate. After the greensheets are stacked, heat and pressure are applied to the greensheets to provide a green ceramic laminate with continuous conductive metal wiring whose layers will remain contiguous during subsequent processing.
This process of applying heat and pressure to the stacked greensheets is called lamination. The green ceramic laminate is then fired in a process called sintering, where the green laminate is densified under heat and pressure. The process of sintering ceramic under uniaxially applied pressure is also known as hot pressing. When the pressure is applied in all directions, then the sintering process is typically known as hot isostatic pressing. In contrast, free sintering typically refers to the process of sintering under no external load or pressure.
During the hot pressing process, employed primarily for densifying the ceramic and the conductive metal materials in MLC substrates, large volume shrinkage of the MLC substrate typically occurs. More specifically, in the case of hot pressing, when the pressure is applied in one direction, the volume shrinkage experiences significant non-uniform viscous deformation throughout the densifying body. Since both the densification and viscous deformation processes are typically dependent on the sample viscosity, these two processes happen simultaneously but at different deformation rates which are temperature sensitive. In addition, when hot pressing MLC products, the densification process will also be dependent on the distribution of metal phase while being somewhat insensitive to external conditions, mainly because the primary driving force for densification is the ceramic phase surface tension. In contrast, the viscous deformation process will have a strong dependency on all external forces applied to the sample.
In general the ceramic and conductive metal materials have a wide difference in physical and transport properties. The onset of densification and the densification profiles between the ceramic and metal phases differ widely as well. With application of external pressure during the sintering process, some of the differences in densification rates may be reduced when the metal densification rate is sensitive to applied pressure. But the use of uniaxial external pressure during densification creates viscous deformation in the sample as well. The complex densification process of the composite, in conjunction with the viscous deformation rate result in distortion, both in the pattern of the conductive metal features and in the substrate body dimensions.
Distortion is defined as deviation in actual post sinter dimensions from the ideal design dimensions. Distortion in the body dimensions includes deviation in surface flatness called camber. Distortion control in sintering by hot pressing processes requires the conductive metal and the ceramic material to have similar shrinkage rates, the application of external pressure at a rate consistent with the ceramic-metal composite physical properties, and careful selection of the method to apply the pressure to the product. However, even with careful selection of materials, variations in material from lot to lot can result in unpredictable shrinkage due to, for example, contamination or particle size distribution. Further, the application of external pressure to the densifying sample may also introduce processing related variations, such as load variation, which can result in product to product variation on a given sample batch and generate product distortion. In MLC substrates this distortion can manifest itself as substrate warping, substrate camber, and variations in substrate dimensions. High distortion results in product with low yield and increased production costs.
Hot pressing is typically used to densify ceramic-metal composites at lower temperatures than what is needed to complete the same process using a free sintering method. The use of external pressure during densification also helps the control of substrate camber during densification when the difference in shrinkage rate between the ceramic phase and the metal phase is significant and can not be reduced adequately by conventional means such as particle size distribution and material chemistry. In some applications, the use of external pressure is the only manufacturable process feasible to generate a given ceramic-metal composite. But the use of external pressure during sintering introduces many complexities into the sintering process which impact directly on the manufacturing costs.
For example, the use of external pressure during sintering requires the use of specially designed hardware to transfer the pressure to the product under densification. Sintering hardware should not restrict the product heating, cooling, or any chemical reaction involving mass transport, and should not deform significantly under pressure. Also, the hardware used to apply the sintering pressure uses up valuable furnace volume. Thus, higher external sintering pressure and temperature translates directly into more expensive hardware to carry out the already costly sintering process.
Not surprisingly then, the hot pressing process is significantly more expensive than free sintering for a given manufacturing production rate. To reduce cost, each sample being hot pressed may include many final products, which are typically separated in a subsequent post-sinter dicing operation. Unfortunately, the effort to control laminate distortion during hot pressing increases the difficulty significantly when the laminate includes multiple products. This is mainly because in a typical multi-up laminate the space between the individual product samples, or “ups”, is free of metallurgy. The viscoelastic properties of the sintering laminates are dependent on metallurgy distribution and therefore multiup laminate sintering inherently has built in variations in physical and transport properties.
The manufacture of MLC substrates involves multiple processes which directly impact the product dimensions and distortion during the sintering step. Extensive effort is expended at increased cost to control the post sinter MLC substrate dimensions. Advances in microelectronic technology has continuously increased the number of chip input/output “I/O”, while decreasing the corresponding chip size. This creates a demand for MLC substrates with reduced top surface metal (TSM) interconnect dimensions. Correspondingly the MLC substrate bottom surface I/O pad density needs to be increased. Such a design need increases the challenge of product build, in particular product dimensional control. Therefore, there is a need for cost-effective distortion control in MLC substrate manufacturing.
There are a number of methods employed currently to control substrate dimensions during MLC substrate manufacturing that are applicable to ceramic-metal systems which are densified under free sintering conditions. However, methods which can be used when the densification is done under external pressure are limited. Sometimes, an additional sinter process under pressure is applicable and will reduce ceramic distortion in some material systems. However this process is expensive and results in additional product yield loss. Often this process is not possible. Additionally, tailoring the type of conductive metal used throughout the substrate may be employed to control product distortion, but this is not useful to control global distortion.
Also, this solution is not comprehensive and does not always address the problem of individual product distortion. Selective distribution of the conductive metal throughout the individual product, to the maximum extent possible, can bound individual product distortion but fails to control global distortion problems. Greensheet stack lamination pressure adjustment is sometimes used to control global distortion. However this technique is not as effective when used with hot pressing. Finally, product redesign may be used as a tool to reduce distortion in some cases by adjusting the conductive metal distribution in key areas. However, this is undesirable since it is very costly and impacts new product time to market. The existing procedures and models used to control product dimensions are not fully predictive, and are therefore not dependable and quite limiting.
There are methods proposed by others to improve the dimensional control of electronic packages. Natarajan et al. U.S. Pat. No. 6,627,020, the disclosure of which is incorporated by reference herein, discloses the use of discrete non-densifying structures to control the dimensions of a free sintered multilayer ceramic substrate. Robbins et al. U.S. Pat. No. 5,801,073, the disclosure of which is incorporated by reference herein, discloses a method for producing an electronic packaging device made of dissimilar materials within a package. Robbins discloses a method to achieve minimal overall shrinkage of the package by the use of a high purity reaction bonded silicon nitride as a dielectric ceramic material.
Mori et al. U.S. Pat. No. 5,370,760, the disclosure of which is incorporated by reference herein, discloses a method to reduce the distortion of the metallized features in a ceramic laminate during the lamination process prior to sintering. Mori discloses the use of a die assembly, which is a tool, having an outer portion and an inner portion which can compress the outer peripheral portion of the laminate to a higher degree than the central portion of the laminate. This disclosure does not address the control of distortion induced during the sintering process.
Notwithstanding the prior art there remains a need to minimize the external sintering pressure and control the dimensions of MLC substrates already designed, but which fail to meet their post sinter dimensional requirements, and whose overall distortion is not amenable to the existing dimensional control methods.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.